Recently, there have increased needs for sensors for sensing the fever generated with influenza, high-performance infrared sensors used as, e.g., human presence sensors and non-contact temperature sensors, and high-performance strain sensors. Furthermore, using VO2 in those sensors has been studied. VO2 is a material exhibiting such characteristics of a CTR (Critical Temperature Resistor) that resistance abruptly lowers at a certain temperature. Tungsten (W), niobium (Nb), molybdenum (Mo), titanium (Ti), etc. are doped for control of the metal-insulator transition temperature.
However, although the non-doped VO2 exhibits an abrupt a resistance change at about 70° C., a temperature range where the resistance abruptly changes is very narrow. Accordingly, even when the non-doped VO2 is employed in a sensor, the sensor is just usable in an extremely limited temperature range. For that reason, an improvement of making the resistance changeable over a wide temperature range is required in order that the non-doped VO2 can be employed in a wide variety of sensors.
In view of the above-mentioned situations, a thin film of VO2 has been improved by not only doping impurities as described above, but also utilizing strain generated in a substrate, or introducing oxygen defects. However, those methods for improving resistance characteristics are very difficult to control, and raise the problem that the thin film of VO2 cannot be stably produced. For example, the VO2 thin film containing many oxygen defects has the problem that its resistance moderately changes near a room temperature, while a resistance change rate is small and control is difficult to obtain the desired characteristics. In order that the sensor can satisfactorily operate at an optionally selected temperature, e.g., a room temperature, it is demanded to realize a resistor exhibiting a large resistance change rate at the optionally selected temperature over a wider temperature range thereabout.
In relation to the above-described problems, Patent Document 1 discloses a thin film of vanadium oxide in which, assuming the vanadium oxide to be expressed by VOx, a range of x satisfies 1.875<x<2.0. According to the technique disclosed in Patent Document 1, a moderate resistance change depending on temperature can be realized over a wide temperature range by suppressing the metal-insulator transition specific to VO2, and higher CTR characteristics than those of the known TiOx can be realized.
The technique disclosed in Patent Document 1 is effective when trying to realize sensing with an infrared ray over a wide temperature range. However, the disclosed technique accompanies with the problem that, because a resistance change corresponding to a temperature change is small, a difficulty arises in sensing a small temperature change or a small intensity change of the infrared. Therefore, any additional expedient, such as adding an amplification circuit, is required on the circuit side.
Furthermore, when the sensor is used as a human presence sensor operating near the room temperature or as a temperature sensor in a refrigerator, a temperature range where the sensor is used is limited to a certain range, and the sensor is not needed to be usable over an excessively wide temperature range. In such a case, the resistance is desirably changed to a large extent in a relatively narrow temperature range (e.g., 20° C.) about a midpoint temperature. Stated in another way, it is desired that the sensor exhibits a resistance change as large as possible not only at the optionally selected temperature, but also over the optionally selected temperature range thereabout.
On the other hand, Non Patent Document 1 states that the metal-insulator transition temperature of the VO2 thin film can be controlled by doping tungsten as an impurity. Thus, the doping of tungsten causes a tendency to lower the transition temperature, to reduce the resistance change rate attributable to the metal-insulator transition, and to make the metal-insulator transition slightly moderated. In other words, the resistance is more moderately changed with the temperature change. Accordingly, by doping tungsten in a larger amount, it is possible to reduce temperature dependency and to widen a working temperature range.
However, the technique disclosed in Non Patent Document 1 has the problem that, because the resistance change rate reduces and the transition temperature lowers, the resistance change cannot be increased over a certain temperature range about an optionally selected temperature.
Patent Document 1: Japanese Patent No. 2786151
Non Patent Document 1: Keisuke Shibuya, Masashi Kawasaki, and Yoshinori Tokura, “Metal-insulator transition in epitaxial V1-xWxO2 (0≦x≦0.33) thin films”, APPLIED PHYSICS LETTERS 96, 022102 (2010)